Many fertilizers, including those commonly used by homeowners on their lawns, consist of granules or particles of a nutritive material. This nutritive material typically includes a blended combination of nitrogen (N) source, a phosphorus (P) source, and a potassium (K) source. The most expensive component of the fertilizer is usually the nitrogen (N) source, especially if the nitrogen is in a so-called "slow-release" form. The present invention relates to improved granular source materials which can be used in making blended fertilizers, including improved nitrogen sources.
During manufacture, nitrogen (N) sources and other fertilizer source materials used as components in commercial fertilizers are granulated by conventional granulation techniques, prilled, or otherwise formed into particles. As a result, they often consist of a mixture of granules of varying sizes. Since commercial fertilizers come in certain preferred size ranges, it is frequently desirable to separate the granules by size. Any oversized or undersized material can then be "compacted" and/or recycled and processed into granules having the desired size.
Granules made by granulation techniques typically are not perfectly smooth and round. Instead, they are irregular in shape and have small protrusions on their surfaces. Granules made by compaction are even more irregular and often have jagged edges. These protrusions and jagged edges tend to break off during later processing of the fertilizer, such as during storage, handling, transportation, mixing, sizing, elevation, packaging and use. Often clouds of dust form in the area where the material is being processed or handled. In addition, since the material is often transported from the manufacturing plant to the fertilizer blending plant in rail cars, significant degradation of the material can occur during moving and unloading and substantial quantities of dust can be created.
For several reasons, this dust is undesirable and has caused a significant problem in the manufacture and processing of blended fertilizers. First, depending on the source material being used to make the fertilizer, the dust may be toxic and, therefore, may pose a health and safety risk to persons inhaling it and otherwise coming in contact with it. Even if the dust is not toxic, it may be unpleasant or noxious to those who handle or use the fertilizer. Moreover, dust causes significant environmental problems, both in the manufacturing plant and in the fertilizer blending plant where the fertilizer source materials are blended to form commercial fertilizers. The Environmental Protection Agency (EPA), as well as state and local agencies, mandates that such facilities maintain certain acceptable levels of particulate matter in the air. Substantial fines may be imposed if airborne particulates exceed the acceptable limits. To avoid these fines, it is necessary to install dust emission systems and scrubber systems, which filter particulate matter from the air. Such systems are often costly and usually require a significant capital investment. Finally, dust created during processing of the fertilizer is difficult to recover, and significant amounts of material are effectively lost during processing, transportation, and handling.
When the fertilizer source material is urea, it has been proposed to add so-called conditioning or hardening agents to the urea so that when it is granulated the resulting particles are harder and more resistant to abrasion and dust formation. Generally, these conditioning agents are added to the urea solution before it is granulated, that is, "upstream" of the granulator. For example, in U.S. Pat. No. 4,587,358, it is proposed that certain lignin compounds be mixed into the urea solution ("synthesis liquor") or into the urea melt upstream of the granulator. In this way, the hardening material is uniformly dissolved into the urea fertilizer to form a homogeneous mixture prior to granulation.
One problem with such processes is that they must be used during manufacture of the fertilizer source material, since they involve changing the composition of the material itself. Many manufacturers are not equipped to use such processes during manufacture. In addition, such processes have not presently been adapted to the manufacture of fertilizer source materials other than urea.
For example, the popular slow-release nitrogen source isobutylidene diurea (sold by Vigoro Industries, Inc. under the trademark IBDU.RTM.), is manufactured by chemically reacting urea and isobutyraldehyde in a "pug mill" to form IBDU.RTM. granules. For general information concerning the chemical composition and properties of isobutylidene diurea, see M. Hamamoto, Proceedings No. 90 Before The Fertiliser Society, "Isobutylidene Diurea as a Slow Acting Nitrogen Fertiliser and the Studies in This Field in Japan," paper read before The Fertiliser Society in London, Jan. 27, 1966, which is incorporated herein by reference and made a part hereof. Because of present manufacturing methods, there is a significant dust problem with commercially available grades of isobutylidene diurea, as well as other porous, granular fertilizer source materials.
It has been proposed to make a hardened granular agglomerate or prill from fine particles of a nitrogen source. For example, U.S. Pat. No. 4,560,400 proposes using a binder to bind fine particles of a poorly or slightly soluble nitrogen source to form a hardened agglomerate or prill. Such methods are typically used with nitrogen source materials having a certain size. In addition, the agglomeration or prilling machinery and equipment used to make such a hardened granular fertilizer are relatively expensive.
It has also been proposed to coat fertilizer granules with various coating materials to form so-called "slow acting" or "slow release" fertilizers, that is, fertilizers which dissolve more slowly in water and, therefore, provide a longer lasting fertilizing effect in the ground. Thus, U.S. Pat. No. 3,576,613 discloses subcoating powdered fertilizers and then encapsulating the subcoated particles with molten elemental sulfur. Similarly, U.S. Pat. No. 3,365,288 proposes coating granular fertilizer with a drying oily polyene polymer and then drying the coating. In U.S. Pat. No. 4,023,955, fertilizer material is encapsulated in a partially hydrated cement. U.S. Pat. No. 4,142,885 discloses a two-stage process in which granules of a preponderantly water-soluble granular fertilize composition is first subjected to accretive granulation and then coated with a suitable, almost impermeable envelopment, such as a plastic, wax, or molten sulfur. Finally, U.S. Pat. No. 4,082,533 proposes using two water insoluble coatings, the first consisting of masonry cement, and the second consisting of a blend of one or more polymers and a wax.
Because these coatings were not designed to prevent dust formation or harden the substrate, they have several drawbacks. When oil coatings are used, soft and porous materials that break, crush, and abrade do not maintain low dust levels because substantial quantities of the finer particles are not coated by these materials. Other coatings, such as wax and heavy resins, also have drawbacks. First, they are relatively expensive. In addition, sophisticated equipment is usually required to apply these coatings.
Another significant problem which may occur with these coatings is that they tend to dilute the available fertilizer source material in the coated material. For example, if granules of nitrogen source material are surrounded with a coating which does not provide a source of nitrogen, the nutritive value of the material per pound is decreased. As a result, more of the material must be used when it is mixed with the phosphorus and potassium components in making commercial grades of blended fertilizers. In addition, depending on the thickness of the coating, the cost of the material per unit of available nutrient may be significantly increased.
U.S. Pat. No. 4,711,659 discloses attrition resistant controlled release fertilizer particles. These particles include a water soluble central mass of fertilizer particles containing NH.sub.2 groups, a base coating surrounding and chemically bonded to the central mass, and a water insoluble sealing layer surrounding and chemically bonded to the base coating. Because the base coating must be chemically bonded to the central mass and the sealing layer, it is necessary to use specific coating compounds, specific processing conditions, and, preferably, multiple coats of the sealing layer to obtain a suitable controlled release fertilizer.
Accordingly, it is an object of the present invention to provide a particulate or granular fertilizer source material which has been hardened and which has significantly increased resistance to dust formation, thereby helping to solve the economic and environmental problems associated with dust formation.
It is a further object of this invention to provide a process for making such a granular fertilizer source from commercially available grades of fertilizer compounds in an efficient and cost-effective manner using relatively simple and readily available equipment and materials.
It is yet a further object of this invention to provide a suitable hardened coating for granular fertilizers having odd or irregular shapes and protrusions or jagged edges on their surfaces.
It is yet a further object of this invention to provide a hardened coating for granular fertilizers consisting of both fine and course particles.
It is still a further object of this invention to provide a coating which does not substantially dilute the available nitrogen in a nitrogen fertilizer source material. IBDU.RTM. is a registered trademark of Vigoro Industries, Inc. of Chicago, Ill.